CVE-2021-33481
📋 TL;DR
CVE-2021-33481 is a stack-based buffer overflow vulnerability in gocr (optical character recognition software) that allows attackers to execute arbitrary code or cause denial of service. The vulnerability exists in the try_to_divide_boxes() function in pgm2asc.c and affects users processing untrusted PGM image files with vulnerable gocr versions.
💻 Affected Systems
- gocr
📦 What is this software?
Optical Character Recognition by Optical Character Recognition Project
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with the privileges of the gocr process, potentially leading to full system compromise if gocr runs with elevated privileges.
Likely Case
Denial of service (crash) when processing malicious PGM files, potentially disrupting OCR workflows.
If Mitigated
Limited impact if gocr runs in sandboxed environments with minimal privileges and processes only trusted files.
🎯 Exploit Status
Exploitation requires the attacker to supply a malicious PGM file that gets processed by gocr. Proof-of-concept exploit code is publicly available in the bug reports.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Versions after 0.53-20200802
Vendor Advisory: https://security.gentoo.org/glsa/202401-28
Restart Required: No
Instructions:
1. Update gocr to version 0.54 or later using your distribution's package manager. 2. For Gentoo: emerge --sync && emerge -av gocr. 3. For Red Hat-based systems: Use updated packages from official repositories.
🔧 Temporary Workarounds
Restrict PGM file processing
linuxLimit gocr to process only trusted PGM files from known sources
# Implement file validation before processing
# chmod 600 sensitive_pgm_files/*.pgm
Run gocr with reduced privileges
linuxExecute gocr with minimal user privileges using sudo restrictions or containers
# Run as non-privileged user
sudo -u nobody gocr input.pgm
# Or use containers
podman run --user 1000:1000 -v $(pwd):/data gocr /data/input.pgm
🧯 If You Can't Patch
- Implement strict input validation - only allow gocr to process PGM files from trusted sources
- Run gocr in a sandboxed environment with minimal privileges and no network access
🔍 How to Verify
Check if Vulnerable:
Check gocr version: gocr --version 2>&1 | head -1. If version is 0.53 or earlier, you are vulnerable.Check Version:
gocr --version 2>&1 | head -1Verify Fix Applied:
After update, verify version is 0.54 or later: gocr --version 2>&1 | grep -q '0.5[4-9]\|0.[6-9]' && echo 'Patched'📡 Detection & Monitoring
Log Indicators:
- Segmentation fault or crash logs from gocr process
- Unexpected termination of OCR processing jobs
Network Indicators:
- N/A - local file processing vulnerability
SIEM Query:
process.name:"gocr" AND (event.action:"segmentation_fault" OR event.outcome:"failure")